This invention relates in general to a method of adjusting the resonant frequencies of a piezoelectric resonator and, in particular, to a method of decreasing the resonant frequencies of such a resonator either before sealing the resonator into an enclosure or after sealing the resonator into an enclosure or both before and after sealing the resonator into an enclosure.
The prior art methods of adjusting frequencies are reviewed by Thompson in U.S. Pat. No. 3,864,161, issued Feb. 4, 1975, and by R. Fischer and L. Schulzke, in the paper, "Direct Plating to Frequency -- A Powerful Fabrication Method For Crystals With Closely Controlled Parameters," at pages 209 to 213 of the Proceedings of the 30th Annual Symposium on Frequency Control -- 1976. The later reference, in particular, discusses the advantages and disadvantages of the various methods used for adjusting the frequencies of resonators. The methods are: mass loading by electroplating, mass removal by etching, spot plating by metal deposition, dielectric mass loading, mass removal by laser trimming, mass loading by sputtering, chemical reaction from a gaseous atmosphers, and direct plating to frequency.
This invention relates to the class of "dielectric mass loading," which Fischer and Schulzke discuss as follows: "Overplating the resonator and its electrode with a dielectric layer of silicon monoxide keeps its mesa-structure nearly unchanged and therefore permits large trimming ranges without significant changes of the energy trapping conditions with filter resonators. The still unresolved problem seems to be the long-time stability of the dielectric layer, which remains chemically active and thus causes drift effects due to gas absorption and/or oxidation. Until now this method has had only a limited range of application to filter resonators."
In the prior art, the frequencies of resonators are adjusted prior to sealing the resonators. Generally, once the resonators are sealed, the frequencies of the resonators can no longer be further adjusted.